Generally, silver iodobromide emulsions are used for camera-speed sensitivity photographic elements as the sensitive silver halide emulsion. Silver iodobromide grains contain silver iodide in silver bromide crystal lattice in such an amount that the amount of silver iodide is not more than a limiting amount to be dissolved in silver bromide. Such silver iodobromide grains have an iodide content not higher than about 40 mol %. Iodide in silver iodobromide emulsions has the following advantages and disadventages.
Advantageously, it increases the efficiency of latent image formation; increase the amount of light absorbed (inherent absorption of the silver halide); improves the adsorption of additives; and improves graineness.
Disadvantageously, iodide restrains development and inhibits chemical sensitization.
Heretofore, many studies have been done with the view of increasing the advantages and decreasing the disadvantages of silver iodobromide photographs having camera-speed sensitivity. The position where the silver iodide was located in the silver halide emulsion grains was found to be of great importance. The following description is found in G. F. Duffin, Photographic Emulsion Chemistry (The Focal Press 1966) page 18.
"An important factor to be considered is the position of iodide when the emulsions are silver iodobromide emulsions. The iodide can be located chiefly in the central part of its crystal or chiefly on the outer surface thereof. The actual position of the iodide varies depending on the preparation conditions of the emulsions. The position influences clearly the physical and chemical characteristics of the crystal."
In the single jet process (wherein the whole amounts of both an iodide salt and a bromide salt are allowed to exist in a reaction vessel and an aqueous solution of a silver salt is then introduced into the reaction vessel to form silver iodobromide grains), silver iodide is first precipitated out and hence silver iodide tends to concentrate in the center of grains.
In the double jet process (wherein both the iodide and the bromide together with the silver salt are simultaneously introduced into the reaction vessel), the distribution of silver iodide in the grains can be controlled. For example, silver iodide can be uniformly iodide on the outer surfaces of grains or a silver iodide shell having a high silver iodide content can be formed when the amount of the bromide salt to be added is reduced or stopped on the way to the formation of grains and the addition of the iodide salt is continued.
The essentials of forming such non-uniform grains are disclosed in JP-A-58-113927 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"}, JP-A-59-99433, JP-A-60-147727, JP-A-60-14331 and JP-A-61-245151. Such grains show improved photographic characteristics compared to uniform grains merely changing the silver iodide content depending on the position of individual grains (particularly in the interior or exterior of grains).
Many methods are known for improving photographic performance such as changing sensitivity by epitaxial-depositing a silver salt on the chosen surface sites of silver halide host grains. For example, JP-A-58-108526 and JP-A-59-133540 disclose silver halide emulsions wherein a silver salt is arranged on the chosen sites of tabular or non-tabular silver halide host grains.
In the many examples disclosed in these patent specifications, however, a silver salt having relatively high solubility (e.g., silver chloride) is formed on the host grains of silver halide having relatively low solubility (e.g., silver iodobromide) by epitaxial growth. However, it is difficult to epitaxially-grow a silver salt having relatively low solubility (e.g., silver iodobromide having a low silver iodide content) on the host grains of silver halide that have a relatively high solubility (e.g., silver iodobromide having a high silver iodide content). In these patent specifications, it is also necessary that sensitizing dyes or ions, as site indicators, capable of being easily adsorbed on the specific plane of crystal be added before the epitaxial growth of the silver salt.
In the method described in JP-A-62-319740, monodisperse tabular grains are prepared and while a high-contrast gradation can be produced photographic material having excellent high-speed development characteristics cannot be provided.